In the water treatment industry, whether municipal, industrial, or remediation, the continued use of standard carbon products causes the effluent pH to increase relative to the influent water pH and often the effluent water pH exceeds 9. This pH excursion occurs with virgin and reactivated carbon and is independent of the raw material. For example, pH excursions have been identified or associated with activated carbons that are made from bituminous coal, sub-bituminous coal, peat, wood, and coconut. The use of carbon having a reduced contact pH to stabilize the pH in water treatment has become available to assist in overcoming these problems, see, e.g., U.S. Pat. Nos. 5,368,738, 5,368,739 and 5,466,378.
Work with these new modified activated carbons has shown that the carbon surface oxidizes at high temperatures with oxygen or air, at ambient temperature with oxygen or air, or with other oxidants such as hypochlorite, nitric acid, and ozone. With this oxidation, the surface of the activated carbon is changed such that the affinity or adsorption capacity for anions such as sulfate is reduced. The adsorption of these anions has been associated with pH excursions. Problems caused by the pH excursions include reduced throughput due to recycle of the high pH water, down time in operation of dialysis systems as the pH is brought into control, wasting water that is high in pH and does not meet the requirements of reverse osmosis systems for high purity water, and monetary fines for exceeding permitted pH levels in wastewater discharge. Historically, the high pH water is alleviated through excessive back washing of the carbon or neutralization of the water through the use of strong acids such as hydrochloric acid or sulfuric acid or a weaker acid such as carbonic acid. These processes are both time consuming and expensive.
Specific characteristics of pH excursions have been described in U.S. Pat. Nos. 5,368,738, 5,368,739 and 5,466,378. In summary, the patents teach that a pH increase in the effluent water from an activated carbon water treatment system is triggered by the presence of anions such as chloride, nitrate, sulfate which occur naturally in water. The art also teaches that activated carbon characterized by a contact pH about 8.5 to 9.0 will exhibit pH excursions with water containing anions such as those stated above. Furthermore, the higher the carbon contact pH the greater the extent of the excursion. U.S. Pat. Nos. 5,368,738 and 5,466,378 teach that the contact pH of the carbon can be reduced by oxidation at elevated temperature. U.S. Pat. No. 5,368,739 teaches that the carbon contact pH can be reduced by oxidation at or near ambient temperature.
Also, it has been observed that the aluminum concentration in the effluent water from the carbon adsorber is higher than the influent concentration. As such, the aluminum concentration may increase to levels that exceed acceptable guidelines. In these situations, as with the pH excursions, the remediation techniques such as recycling or wasting water or strong acid treatment of the activated carbon can be both time consuming and expensive.
Accordingly, it is an object of the present invention to provide a process for producing a modified activated carbon having a contact pH between 6.0 and 9.0. It is also an object of the invention to provide a process for producing a modified activated carbon to eliminate process related problems that are associated with elevated temperature oxidation, such as reduced carbon yield, and to overcome the long treatment times or high gas volumes that are associated with oxidation at or near ambient temperature. It is a further object of the invention to provide a process for producing a modified activated carbon which is highly efficient and cost effective for use in the prevention of pH excursions in water treatment systems. Finally, it is an object of the present invention to utilize the modified activated carbon to control the aluminum concentration in water treatment systems.